Transform this into a flat surface
Two ways of doing this:


Use that sphere for knife project tool


Cut the sphere in half in Edit Mode
Choose Top View
In Object mode select your sphere, and then select the pane
Go into Edit Mode and Knife Project

Collapse the sphere


Cut the sphere in half in Edit Mode
Select all vertices
Scale it along the Z axis to 0 (On a keyboard: S + Z + 0)



I am quite new to Blender myself, but in my experience, these are the two easy ways of achieving it.